The present invention relates to ultrasonic imaging, and more specifically to a mechanical sector scan ultrasonic imaging apparatus.
A known mechanical sector scan ultrasonic imaging apparatus has a hand-held probe which is constructed of a rotatably mounted electroacoustic transducer driven by a motor and an ecoder for generating pulses at a frequency variable as a function of the speed of rotation of the transducer. A typical example of the transducer comprises three piezoelectric elements mounted 120 degrees apart on the circumference of a disc driven by the motor. The transducer elements are sequentially connected to a transmit-receive unit for transmission of a beam of acoustic burst energies over a sector field and reception of acoustic echos returning from acoustic discontinuities in the sector field. A scan converter transforms the sector scan format of the received echo signals into a line-by-line rectangular scan format to enable the signals to be displayed on a video screen of a display unit. The speed of rotation of the motor is basically determined in relation to the horizontal line interval of the raster scan so that a predetermined number of horizontal scans occurs during each revolution of the motor for a particular mode of operation. The interval between successive transmissions is also determined in relation to the operational mode so that it is an integral multiple of the horizontal scan interval. To precisely maintain the motor speed in relation to the number of scan lines and hence to the transmission interval for a given mode of operation, the horizontal sync pulses of the raster scan and the encoder pulses are scaled in number with different dividing factors so that the pulses of each signal generated during a unit time coincide in number with those of the other. Phase comparison is made between the two signals to produce a speed correction signal which is summed with the signal that basically determines the motor speed.
Since the number of pulse-to-pulse phase comparisons is reduced with respect to the number of acoustic bursts generated for each revolution of the transducer, the speed correction signal has not sufficient degree of resolution for the speed of rotation of the transducer. The conventional imaging apparatus thus suffers from erratic speed variations which result in blurred or distorted images on the display screen.